It is recognized that film electrodes on zirconia type exhaust gas sensor bodies should be porous and adherent. U.S. Pat. No. 3,978,006, Topp et al, discloses that catalytically active film electrodes can be applied by a variety of thin layer techniques, such as thermal vaporization, cathode atomization, gas phase deposition, chemical reduction, galvanic deposition, and by applying a platinum suspension with a brush. It also discloses heat treating such electrodes to make them porous after deposition.
My concurrently filed U.S. patent application Ser. No. 177,561, entitled "Surface Etching Before Electroding Zirconia Exhaust Gas Oxygen Sensors", which is also assigned to the assignee of this invention, describes a zirconia pretreatment process that enhances porosity and adherence of deposited electrodes. Unlike sputtered electrodes, evaporated electrodes do not have an appreciable porosity as deposited. Accordingly, it is highly desirable to pretreat the zirconia surface if the electrodes are to be produced by evaporation or the like.
In this invention I found an apparatus and method for providing highly porous adherent electrodes as deposited. In addition, the electrode metal is extremely uniformly distributed over the zirconia surface regardless as to the contour of the zirconia surface, which enhances obtaining more consistent electrode properties among groups of sensors.